Recently, as the electrical components for an automobile increases, electric power generated only by a conventional 12-V low-voltage battery is not sufficient as the power supply on the automobile. Accordingly, a power supply system for vehicle has been proposed that is equipped with two battery systems: a 12-V low-voltage battery and a 36-V high-voltage battery. For example, as disclosed in Japanese Patent Laid-open No. 2001-28832, in a well-known power supply system for vehicle, a low-voltage battery and a high-voltage battery are electrically connected to each other via a DC/DC converter, and the generator normally charges the 36-V battery and then charges the 12-V battery via the DC/DC converter. Furthermore, for example, as disclosed in Japanese Patent Laid-open No. Hei 11 (1999)-299123, in a well-known power supply system for vehicle, a spare battery is provided as a precaution when the regulating circuit that controls direct current supplied to the field coil of the A.C. generator malfunctions, and if the malfunction occurs, the spare system starts to generate electric power so that the vehicle can continue traveling.